Fuel Systems: Emission Control

Creation of emissions	Sources of emission	Emissions from a fuel-driven motor vehicle usually come from four sources: the fuel tank, the carburetor, the crankcase, and the exhaust system.
	Combustion	To reduce exhaust emissions, some devices or systems control the combustion process itself, while others treat the resulting exhaust gases. Most modern vehicles use a combination.
	Combustion chamber design	Combustion chamber design can affect the combustion process, and the level of emissions. Designs that increase gas flow rate and promote vaporisation, distribute fuel more evenly in the chamber.

Types of emissions	Hydrocarbons	Hydrocarbons are a major component of motor vehicle emissions. Hydrocarbon emissions react with other compounds in the atmosphere to produce photo-chemical smog.
	Hydrocarbons in exhaust gases	Hydrocarbon emissions in the exhaust gases can be caused by valve overlap, incomplete combustion and ignition system misfiring.
	Oxides of nitrogen	Under the high temperatures and pressure of combustion, nitrogen from the air combines with oxygen to produce oxides of nitrogen. Oxides of nitrogen can be produced in any operating internal combustion engine.
	Particulates	In spark-ignition engines, particulates are caused by incomplete combustion of rich air-fuel mixtures. In compression-ignition engines, they are caused by a lack of turbulence and lack of oxygen.
	Carbon monoxide	Carbon monoxide is produced during combustion when there are not enough oxygen molecules around the hydrocarbon molecules. This can be caused by an incorrect air-fuel ratio. Carbon monoxide is an extremely poisonous gas.
	Carbon dioxide	When complete combustion of air and fuel occurs, carbon dioxide and water are produced.
	Sulfur content in fuels	Gasoline and diesel fuels contain sulfur as part of their chemical makeup. When combined with water vapor, sulfuric acid is produced.

Emission control	Evaporation emission control	A positive crankcase ventilation system flushes vapors from the crankcase into the intake manifold, to join with the inlet air-fuel mixture. Once there, the vapors are drawn into the engine for burning.
	Catalytic conversion	Maintaining the stoichiometric ratio is necessary for a catalytic converter to operate efficiently. It receives all the engine's exhaust gases, and chemically converts remaining pollutants to less harmful substances.
	Closed loop	A closed loop is part of a feedback system that collects information on how a system is operating & feeds that information back to affect how the system is working.
	Regulated emissions	Air pollutants are classified as either primary or secondary contaminants. Typically the regulated emissions are carbon monoxide, hydrocarbon, nitrogen oxides and particulate matter.
	Crankcase emission control	A PCV valve, controlled by manifold pressure, regulates gas flow between the crankcase and the inlet manifold. With the engine off, the valve is closed. Air cannot enter the inlet manifold. This allows the engine to start.
	EGR valves	The EGR valve lets some exhaust gases pass into the intake system. During combustion, these exhaust gases absorb heat. This lowers peak combustion temperatures, and reduces formation of oxides of nitrogen.
	Controlling air-fuel ratios	Electronic fuel injection and engine management systems deal with emissions more effectively than carbureted engines by more closely controlling the air-fuel ratio entering each cylinder, and by ensuring the ignition timing matches operating conditions.
	Charcoal storage devices	A charcoal canister can store large quantities of fuel vapor. It has connections for the fuel tank vent line, the atmosphere, and the purge line, which carries the vapors to the intake manifold.

On-board diagnostics	OBD systems	On-Board Diagnostic systems use the vehicle's computers to detect problems with its emission components and other systems.
	Diagnostic trouble codes	The diagnostic trouble codes inform the technician of the computers opinion of the location of a system fault.
	Monitoring emissions	OBD II systems monitor the emission systems in a vehicle either 'continuously' or 'non-continuously'.

Emissions procedures	Checking & cleaning a PCV valve	Most modern vehicles have a PCV valve, which should be checked periodically to make sure that it is not clogged with deposits from the exhaust waste. The objective of this procedure is to show you how to check and clean the PCV valve part of the emission control system.
	Obtaining & interpreting scan tool data	The objective of this procedure is to show you how to retrieve, record and clear stored OBD I & II diagnostic trouble codes using a scan tool. Remember, when retrieving codes for an OBD I vehicle, clear the codes then start the vehicle and recheck for any codes that reset. When diagnosing and OBD II vehicle, do not clear the codes until the vehicle is repaired.

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